Buggy code:
public abstract class AbstractSyntaxParser extends AbstractParser { protected static Map < Case , Set < Case > > caseMatchMap = new HashMap < Case , Set < Case > > ( ) ; protected static Map < SingularPlural , Set < SingularPlural > > spMatchMap = new HashMap < SingularPlural , Set < SingularPlural > > ( ) ; private static PrepConjRegistry prepConjRegistry ; protected static final UnaryMatcher < SyntaxToken > prepMatch = AbstractSyntaxParser . hasAny ( PREP ) ; protected static final UnaryMatcher < SyntaxToken > prepConjMatch = AbstractSyntaxParser . hasAny ( PREP , CONJ ) ; protected static final UnaryMatcher < SyntaxToken > adjectiveMatch = AbstractSyntaxParser . hasAny ( ADJF ) ; protected static final UnaryMatcher < SyntaxToken > nounMatch = AbstractSyntaxParser . hasAny ( NOUN ) ; protected static final UnaryMatcher < SyntaxToken > adverbMatch = AbstractSyntaxParser . hasAny ( ADVB ) ; protected static final UnaryMatcher < SyntaxToken > participleMatch = AbstractSyntaxParser . hasAny ( PRTF ) ; protected static final UnaryMatcher < SyntaxToken > verbMatch = AbstractSyntaxParser . hasAny ( VERB , INFN ) ; protected static final UnaryMatcher < SyntaxToken > nproMatch = AbstractSyntaxParser . hasAny ( NPRO ) ; protected static final UnaryMatcher < SyntaxToken > verbLikeMatch = AbstractSyntaxParser . hasAny ( VERB , INFN , PRTF , PRTS , GRND ) ; public AbstractSyntaxParser ( AbstractWordNet wordNet ) { } protected AbstractWordNet wordNet ; public boolean isIterative ( ) { } protected static < T extends Grammem > Map < T , T > matchGrammem ( Set < T > set0 , Set < T > set1 , Map < T , Set < T > > matchMap ) { } @ SuppressWarnings ( "unchecked" ) protected static < T extends Grammem > void fillGrammemMap ( Set < T > all , Map < T , Set < T > > map ) { } protected boolean checkParents ( IToken newToken , List < IToken > children ) { } protected List < IToken > collectParents ( IToken ch ) { } protected static SyntaxToken combineNames ( SyntaxToken mainGroup , SyntaxToken token , int tokenType ) { } private static Set < AnimateProperty > matchAnimatedProperty ( GrammemSet gs0 , GrammemSet gs1 ) { } public static Set < Gender > matchGender ( GrammemSet gs0 , GrammemSet gs1 ) { } protected static Set < Gender > matchGender ( Set < Gender > set0 , Set < Gender > set1 ) { } public static boolean matchSP ( SyntaxToken token0 , SyntaxToken token1 ) { } public static boolean matchSP ( List < GrammemSet > grammemSets0 , List < GrammemSet > grammemSets1 ) { } public static Map < SingularPlural , SingularPlural > matchSP ( GrammemSet gs0 , GrammemSet gs1 ) { } protected static Map < SingularPlural , SingularPlural > matchSP ( Set < SingularPlural > set0 , Set < SingularPlural > set1 ) { } public static Map < Case , Case > matchCase ( GrammemSet gs0 , GrammemSet gs1 ) { } protected static Map < Case , Case > matchCase ( Set < Case > set0 , Set < Case > set1 ) { } protected boolean checkIfAlreadyProcessed ( SyntaxToken ... tokens ) { if ( ( tokens == null ) || ( ( tokens . length ) < 2 ) ) { return false ; } List < IToken > commonParents = collectParents ( tokens [ 0 ] ) ; if ( ( commonParents == null ) || ( commonParents . isEmpty ( ) ) ) { return false ; } for ( int i = 1 ; i < ( tokens . length ) ; i ++ ) { List < IToken > parents = collectParents ( tokens [ i ] ) ; if ( ( parents == null ) || ( parents . isEmpty ( ) ) ) { return false ; } List < IToken > toRemove = new ArrayList < IToken > ( ) ; for ( IToken t : commonParents ) { if ( ! ( parents . contains ( t ) ) ) { toRemove . add ( t ) ; } } <START_BUG> commonParents . retainAll ( toRemove ) ; <END_BUG> if ( commonParents . isEmpty ( ) ) { break ; } } boolean result = ! ( commonParents . isEmpty ( ) ) ; return result ; } protected boolean isContained ( IToken token0 , IToken token1 ) { } public static final < T extends Grammem > UnaryMatcher < SyntaxToken > hasAll ( T ... tran ) { } public static final < T extends Grammem > UnaryMatcher < SyntaxToken > has ( T infn ) { } public static final UnaryMatcher < SyntaxToken > not ( final UnaryMatcher < SyntaxToken > infn ) { } public static final UnaryMatcher < SyntaxToken > and ( UnaryMatcher < SyntaxToken > ... matchers ) { } public static final UnaryMatcher < SyntaxToken > or ( UnaryMatcher < SyntaxToken > ... matchers ) { } public static final UnaryMatcher < SyntaxToken > hasAny ( Grammem ... gf ) { } public static final UnaryMatcher < SyntaxToken > createCaseMatcher ( Collection < Case > cases ) { } public static final < T extends Grammem > UnaryMatcher < SyntaxToken > hasAny ( Set < T > set ) { } protected boolean isPrepOrConj ( IToken newToken ) { } protected MeaningElement getPreposition ( IToken newToken ) { } protected MeaningElement getConjugation ( IToken newToken ) { } protected PrepConjRegistry getPrepConjRegistry ( ) { } public boolean isRecursive ( ) { } protected boolean isComma ( IToken newToken ) { } public static boolean matchParticiple ( IToken token ) { } }

Fix:
commonParents . removeAll ( toRemove ) ;